Underwater low frequency sonic communication

ABSTRACT

This invention relates to very low frequency sound wave signaling and communication through sea water over long distances.

United States Patent [191 Tyrrell [54] UNDERWATER LOW FREQUENCY SONIC COMMUNICATION [75] lnventor: Warren A. Tyrrell, Mendham, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: May 31, 1963 [21] Appl. No.2 285,556

[52] US. Cl. 340/5 R, 340/3 FM [51] Int. Cl. 1104b 11/00 [58] Field of Search 340/3, 5, 6, 5 T, 15, 3 FM,

340/3 R, 5 R; 181/.51, 51; 343/l7.5; 325/28,

[451 May 14, 1974 [56] References Cited UNITED STATES PATENTS 2,994,060 7/1961 Ross 340/3 R 3,016,513 1/1962 Van Dyke 340/3 R Primary ExaminerRichard A. Farley Attorney, Agent, or Firm-Richard S. Sciascia; Arthur A. McGill [57] ABSTRACT This invention relates to very low frequency sound wave signaling and communication through sea water over long distances.

12 Claims, 6 Drawing Figures UNDERWATER LOW FREQUENCY SONIC COMMUNICATION The sound wave energy propagation characteristics of sea water are such that for transmission of intelligence on sound wave energy over extended distances, e.g., several hundred miles or more, it is necessary to use very low frequency sound wave energy, i.e., below 1,000 cycles per second, generally a fraction of 1,000 cycles per second. However, a major difficulty is that there is considerable sound wave energy in the sea in that band originating from a variety of sources, that can interfere with, mask, or drown out a very low frequency signal. In Fundamentals of Sonar," by .l. W. Horton, published by US. Naval Institute, Annapolis, Md., pages 57-72 there is a discussion of various kinds of sound wave energy in the sea. The biggest part of the interfering noise is often self noise, that is, the noise originating from the platform to which the receiver is connected. Any intelligence imparted to the water on very low frequency sound wave energy must be of a character clearly discernible from self noise at the receiver and other sound wave energy present in the sea in the very low frequency band. Also, projected signal power must be adequate for long range transmission.

An object of this invention is to transmit a signal or a message through sea water over long distances.

A further object is to impart a very low frequency sound wave or acoustic signal to sea water of frequency-time distribution and character that is readily discernible from self noise at the receiver platform and from sound wave energy in the sea in the same-frequency band and having frequency components in common with the signal and that are of comparable or higher power level.

A further object is to provide a practical, reliable, and efficient method and apparatus for communicating over long distances through sea water with very low frequency sound wave signals that are readily discernible at the detection site from interfering sound wave energy, including self noise at the detection site, in the same frequency band and having frequency components of comparable or higher power level.

Other objects and advantages will appear from the following description of an example of the invention, and the novel features will be particularly pointed out in the appended claims.

FIG. 1 is a block diagram showing underwater acoustic signal receiving equipment for use in practising this invention;

FIG. 2 illustrates a recording that might be obtained with the equipment shown in FIG. 1;

FIG. 3 is a block diagram of an underwater acoustic signal transmitter;

FIGS. 4 and 5 illustrate recordings of signals obtainable by use of the equipment shown in FIGS. 1 and 3; and

FIG. 6 is a block diagram of another novel equipment for use in practising this invention.

In FIG. 1, there is shown a portion of the equipment used in practising this invention, including a low frequency hydrophone 10 for sensing waterborne sound wave energy in a selected frequency band below 1,000 cycles per second coupled by amplifiers and other conventional circuits, not shown, to a spectrum analyzer 12. The spectrum analyzer is of the type that records frequency-time distribution of input energy on a paper record 14, as shown in FIG. 2. The length dimension of the recording on the paper record obtained from the spectrum analyzer is a linear time base and the width of the paper record is a linear frequency ordinate between predetermined frequency limits f and 1;. Several longitudinal lines are shown on the record representing energy components of several frequencies and of different power levels. If a component frequency is constant the line is straight; discontinuities such as l6, l8, and 20 represent frequency fluctuation, start of a frequency component and termination of frequency component respectively. The recording is plotted line-byline along the width dimension as on a facsimile machine; broken line 22 in FIG. 2 represents the path of a marking or recording element in one traverse across the paper record..The width of the paper record may be on the order of 6 inches and the paper record may be advanced past the recording position stepwise on the order of 12 inches per hour; however, both these parameters may be considerably larger or smaller. In US. Pat. No. 2,996,667, there are described spectrum analyzers suitable for use in the practice of this invention.

In FIG. 3, there is shown additional equipment for use in practicing this invention including an oscillator 24 having a manually operable frequency changer knob 26 for use in sweeping the frequency of the oscillator between f and f and any two frequencies therebetween. A power amplifier coupled to the oscillator drives the projector transducer 30 at the oscillator frequency. A switch mechanism 29 or remotely operable relay is provided to selectively couple or uncouple the transducer and power amplifier. A frequency meter 32 is coupled to the power amplifier. With the guidance of a stop watch and after a period of training, one may operate the frequency changer 26, observe the frequency meter 32 and the stop watch, not shown, and thereby change the frequency generated by the oscillator from a selected starting frequency between f and f at a selected rate that is approximately constant to a selected terminating frequency between f and f If the frequency is changed from f to f and the elapsed time for the change is t, and some of the acoustic power projected by transducer 30 is intercepted by hydrophone 10, the paper record 14 of the spectrum analyzer will register the signal as an approximately straight skew line, such as 34 of FIG. 4. A signal exemplified by the line 34 is readily discernible from other acoustic signals recorded on paper record 14'. Using the same method but starting at f and terminating'at f,, a signal 35 of opposite slope is recorded on the paper record. The signal is more easily discernible as the shape of the line 34 approaches rectilineari ty, particularly where the line 34 is pale, in comparison to other acoustic signals recorded on the paper record. While the described method may not produce a skew line that is absolutely straight, if the frequency changer 26 has a substantial gear ratio and the operator is trained, a good approximation of a straight line is recorded.

Laboratory evidence and experience supports this slant line format as an optimum underwater communication code. There is a well defined relationship between the detectability of a line on the record and the length or duration of the line. Theoretical analysis indicates that the detection threshold of a straight line on such a record is improved 1.5 db for every doubling of I line length. From extensive laboratory studies, I have established that the improvement is closer to 2 db per doubling of line length. Hence, the choice of the length of individual characters in a code is intimately related to the prevailing signal levels and noise background. However, if the line is not approximately straight, the advantage gained by line lengthening toward discerning the line against the recorded background noise may be largely lost.

In place of a stop watch or a clock, a synchronous motor may be attached or integrated with the frequency meter 32, to propel an indicator or sweep hand along the same path that the frequency meter indicator traverses. With this arrangement a more constant rate of change of frequency is obtainable, in that a trained operator by observing the sweep hand while manually controlling the frequency changer can cause the frequency meter indicator to track with the sweep hand.

The oscillator 24 may be provided with a push button band selector, e.g., as in a radio receiver that rapidly shifts the oscillator frequency to the starting frequency for a subsequent signal. If the width of the paper record is divided into two or more frequency bands, e.g.,f "f f 'f f 'f as shown in FIG. 5, by imprinting on the paper record one or more longitudinal band separation lines 36 and 38 parallel to the edges of the paper record, a message can be transmitted in accordance with the above described method including six distinct signal characters displaced in time along the paper record. A four-push-button band switch on the oscillator would be used to select the starting frequency for each signal character and the manually operated frequency changer would be operated .in either direction to sweep across the frequency band at the selected constant rate. A message of threecharacters in accordance with these principles is shown as lines 40, 42, 44 in FIG. 5 which cover approximately equal periods of time. The time for recording each of the lines 40, 42, 44, requires several minutes. When the band switch is operated at the end of one signal to switch the oscillator frequency to the starting frequency for the next signal, no mark is recorded on the paper record because the band switching action is faster than the required response time of the spectrum analyzer recorder.

Instead of operating the frequency changer manually a synchronous motor and a reversible clutch may be used to selectively drive the frequency changer in either direction and to uncouple the frequency changer and the synchronous motor. Equipment of this type is commercially available. Catalog P of General Radio Corporation, April 1959, pages 149-152 contain ads for this type of equipment.

In FIG. 6 there is shown another apparatus for generating the signal energy discussed above. This apparatus includes a conventional gasoline engine 50 drivingly coupled to a conventional alternator 52. The output power of the alternator is coupled to the projector transducer as in FIG. 3. A conventional type electric speed governor 54 of the type commercially marketed, e.g., by Westinghouse, controls the engine throttle in conventional manner to control the engine speed. Basically, the governor compares two signals, one signal being at the output frequency of the alternator which is changed to a voltage directly proportional to the signal frequency, and the other signal is a reference voltage. It compares the signals and adjusts the engine throttle for reducing difference or error voltage to zero.

In FIG. 6, the reference voltage is obtained from a manually controllable source of reference signal 56 having a knob 58. The reference signal is adjusted by any of the methods described previously. Alternatively, a programmed reference voltage may be fed to the governor to cause the alternator frequency to follow any message program within the capability of the engine alternator, i.e., frequency band of the engine alternator and the rate of change of frequency which the engine alternator can follow. Each message character to be formed consists of a slow linear change of frequency with time, e.g., a change of several cycles per second linearly over a period of several minutes. With the apparatus described a change of engine speed to effect a frequency change at least 30 cycles per second can be achieved in a time sufficiently brief as to produce substantially no registration on the paper record. This enables this equipment to generate several distinct message characters each having a bandwith of several cycles per second.

Depending upon requirements such as power frequency band and rate of change of frequency within the operating band, all components of the engine alternator system either are available as standard commercial items, or can be obtained by straight forward extensions of or modifications to standard commercial designs.

Some major advantage of the engine-alternator system are economy, simplicity, high power capability, reliability, and compactness plus the fact that it combines the functions of power generation and signal generation. Engine-alternator combinations can be designed and assembled to provide acoustic power output as high as any anticipated requirements and is economical in space requirements.

One characteristic of the engine-alternator system that may be a disadvantage in some circumstances is that once the components of the system are designed or matched for a selected set of operating parameters, e.g., center frequency, band-width, etc., a specification change on the order of 15-25 percent in these parameters would render the engine alternator combination unusable. An electronic system as in FIG. 3 is better able to tolerate change in operating parameters.

It will be understood that various changes in the details, materials and arrangements of parts (and steps), which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

I claim:

1. A method of signalling between locations in the sea that are spaced so far apart that acoustic energy below 1,000 cycles per second is far superior to acoustic energy of higher frequency in respect to attenuation in propagation between said locations comprising:

a. driving an elongate paper record longitudinally through a narrow zone transverse to the path of the record at a constant rate whereby the length dimension of the paper record that passes through said zone provides a linear time base,

b. continuously sensing at one of said locations acoustic energy in a predetermined frequency band below 1,000 cycles per second incident to said one location,

c. and registering the frequency components of the sensed energy as marks on the paper record in said transverse zone at distances from one edge of the paper record that are a linear function of frequency within said band,

d. whereby frequency components in the incident energy continuing for various periods of time are recorded as longitudinal lines along the paper record graphically indicating the frequency-time distribution of the acoustic energy in said frequency band,

e. projecting into the sea at another of said locations a swept frequency acoustic signal of sufficient power to reach and be sensed at said one location, the frequency limits of said signal being within the frequency band sensed at said one location, the swept frequency signal occurring over a time interval of sufficient length to record on the paper record as a transverse skew line which is readily discernible from other generally longitudinally lines recorded along the paper record. 2. A method of signalling between stations in the sea as defined in claim 1, including:

a. projecting into the sea a plurality of said swept frequency acoustic signals in succession wherein l. at least one of said signals progresses from a starting frequency within said band to a higher frequency within said band and 2. wherein the frequency of at least another of said signals progresses from a starting frequency within said band to a lower frequency within said band. 3. A method of signalling between stations in the sea as defined in claim 1, wherein:

a. the frequency of each signal changes at a rate which is approximately linear with time. 4. A method of signalling between locations in the sea as defined in claim 1, including:

a. projecting into the sea a plurality of said swept frequency acoustic signals in succession I. wherein the frequency limits of at least one of said signals are within the lower half of said frequency band and 2. wherein the frequency limits of at least another of said signals are within the upper half of the frequency band. 5. A method of signalling between locations in the sea as defined in claim 4,

a. wherein at least one of said signals progresses from a starting frequency to a higher frequency and b. wherein at least another of said signals progresses from a starting frequency to a lower frequency. 6. A method of signalling between locations in the sea as defined in claim 5, further including a. cyclically scanning at a constant rate the sensed acoustic energy in said frequency band from one frequency limit to the other frequency limit of the band and b. cyclically traversing the width dimension of the paper record in said zone edge to edge in unison with the scanning of the acoustic energy c. and registering a mark along the width dimension of the paper record in said zone for each frequency component in the scanned acoustic energy. 7. A method of signalling between locations in the sea as defined in claim 6,

a. wherein the time interval between successive scans is a negligible percentage of the time for each scan.

8. A method of signalling between locations in the sea as defined in claim 1, further including a. cyclically scanning ata constant rate the sensed acoustic energy in said frequency band from one frequency limit to the other frequency limit of the band b. cyclically traversing the width dimension of the paper record in said zone, edge to edge, in unison with the scanning of the acoustic energy and c. registering a mark along the width dimension of the paper record in said zone for each frequency component in the scanned acoustic energy.

9. A method of signalling between locations in the sea comprising a. projecting into the sea at one of said locations a swept frequency acoustic signal between predetermined frequency limits,

b. continuously sensing incident acoustic energy between said predetermined frequency limits at another of said locations,

c. graphically recording on rectangular coordinates the sensed energy as linear functions of time and I frequency d. whereby the swept frequency signal recording is readily discernible as skew to recordings of frequency components of other sensed acoustic energy in the sea.

10. A method of signalling between spaced locations submerged in the sea comprising a. continuously sensing acoustic energy incident to one of the stations, y

b. driving an elongate paper record longitudinally at a fixed rate to provide a linear time base along the paper record,

c. analyzing the sensed acoustic energy for the frequency components between predetermined frequency limits,

d. marking the paper record as it passes a transverse stationary reference position to represent the frequency components obtained by the analysis along the transverse dimension of the paper record as frequency ordinate,

e. whereby various generally longitudinal registrations are recorded along the paper record presenting the frequency-time distribution of acoustic energy sensed in the water, V

f. projecting into the sea at the other of said stations an acoustic signal of sufficient power to reach said one location and characterized by a swept frequency band, which at least for the most part, is in common with the frequency band sensed at and recorded by said one station,

g. the swept frequency signal occurring over a time,

interval long enough to record on the paper record as a transverse skew registration which is readily discernible from other generally longitudinal registrations along the paper record.

11. A method of transmitting a message from one location to another location both of which are submerged in the sea and many miles apart, comprising at said another location a. continuously sensing acoustic energy in a very low frequency band characterized by long distance propagation by the seawater,

b. cyclically scanning and analyzing said band for spectral distribution,

c. facsimile recording the spectral distribution in phase with the cyclic scanning and analyzing to present the frequency-time distribution of acoustic energy in said band sensed in the water,

. projecting into the sea at said one location a series the time span of each swept frequency signal being such that the signals record generally transverse to and are discernible from registrations of pulse-like transitory noise and registrations of ship noise and other acoustic energy that is comparatively constant in frequency character for extended periods.

12. A method of signalling between spaced stations in the sea comprising a. projecting into the sea at one of said stations acoustic power characterized by progressively changing frequency from a selected starting frequency to a selected terminating frequency at an approximately regular rate in a predetermined time interval,

b. the projected power frequency being below 1,000 cycles per second for propagating the signal over a substantial range and the time interval for the change being at least one minute,

0. continuously sensing at the other station acoustic energy reaching the other station between band limits that include the swept frequency of said projected acoustic power,

d. driving an elongate paper record longitudinally at a fixed rate and recording along the paper record, using the length of the paper record as a time base and the width of the paper record as a frequency ordinate, the frequency distribution of acoustic energy in said band e. whereby any sensed acoustic energy at'relatively constant frequency records along the paper record as longitudinal linear registrations and the swept frequency signal records as a linetransverse to the length of the paper record and readily discernible from other registrations on the paper record. 

1. A method of signalling between locations in the sea that are spaced so far apart that acoustic energy below 1,000 cycles per second is far superior to acoustic energy of higher frequency in respect to attenuation in propagation between said locations comprising: a. driving an elongate paper record longitudinally through a narrow zone transverse to the path of the record at a constant rate whereby the length dimension of the paper record that passes through said zone provides a linear time base, b. continuously sensing at one of said locations acoustic energy in a predetermined frequency band below 1,000 cycles per second incident to said one location, c. and registering the frequency components of the sensed energy as marks on the paper record in said transverse zone at distances from one edge of the paper record that are a linear function of frequency within said band, d. whereby frequency components in the incident energy continuing for various periods of time are recorded as longitudinal lines along the paper record graphically indicating the frequency-time distribution of the acoustic energy in said frequency band, e. projecting into the sea at another of said locations a swept frequency acoustic signal of sufficient power to reach and be sensed at said one location, the frequency limits of said signal being within the frequency band sensed at said one location, f. the swept frequency signal occurring over a time interval of sufficient length to record on the paper record as a transverse skew line which is readily discernible from other generally longitudinally lines recorded along the paper record.
 2. wherein the frequency limits of at least another of said signals are within the upper half of the frequency band.
 2. wherein the frequency of at least another of said signals progresses from a starting frequency within said band to a lower frequency within said band.
 2. A method of signalling between stations in the sea as defined in claim 1, including: a. projecting into the sea a plurality of said swept frequency acoustic signals in succession wherein
 3. A method of signalling between stations in the sea as defined in claim 1, wherein: a. the frequency of each signal changes at a rate which is approximately linear with time.
 4. A method of signalling between locations in the sea as defined in claim 1, including: a. projecting into the sea a plurality of said swept frequency acoustic signals in succession
 5. A method of signalling between locations in the sea as defined in claim 4, a. wherein at least one of said signals progresses from a starting frequency to a higher frequency and b. wherein at least another of said signals progresses from a starting frequency to a lower frequency.
 6. A method of signalling between locations in the sea as defined in claim 5, further including a. cyclically scanning at a constant rate the sensed acoustic energy in said frequency band from one frequency limit to the other frequency limit of the band and b. cyclically traversing the width dimension of the paper record in said zone edge to edge in unison with the scanning of the acoustic energy c. and registering a mark along the width dimension of the paper record in said zone for each frequency component in the scanned acoustic energy.
 7. A method of signalling between locations in the sea as defined in claim 6, a. wherein the time interval between successive scans is a negligible percentage of the time for each scan.
 8. A method of signalling between locations in the sea as defined in claim 1, further including a. cyclically scanning at a constant rate the sensed acoustic energy in said frequency band from one frequency limit to the other frequency limit of the band b. cyclically traversing the width dimension of the paper record in said zone, edge to edge, in unison with the scanning of the acoustic energy and c. registering a mark along the width dimension of the paper record in said zone for each frequency component in the scanned acoustic energy.
 9. A method of signalling between locations in the sea comprising a. projecting into the sea at one of said locations a swept frequency acoustic signal between predetermined frequency limits, b. continuously sensing incident acoustic energy between said predetermined frequency limits at another of said locations, c. graphically recording on rectangular coordinates the sensed energy as linear functions of time and frequency d. whereby the swept frequency signal recording is readily discernible as skew to recordings of frequency components of other sensed acoustic energy in the sea.
 10. A method of signalling between spaced locations submerged in the sea comprising a. continuously sensing acoustic energy incident to one of the stations, b. driving an elongate paper record longitudinally at a fixed rate to provide a linear time base along the paper record, c. analyzing the sensed acoustic energy for the frequency components between predetermined frequency limits, d. marking the paper record as it passes a transverse stationary reference position to represent the frequency components obtained by the analysis along the transverse dimension of the paper record as frequency ordinate, e. whereby various generally longitudinal registrations are recorded along the paper record presenting the frequency-time distribution of acoustic energy sensed in the water, f. projecting into the sea at the other of said stations an acoustic signal of sufficient power to reach said one location and characterized by a swept frequency band, which at least for the most part, is in common with the frequency band sensed at and recorded by said one station, g. the swept frequency signal occurring over a time interval long enough to record on the paper Record as a transverse skew registration which is readily discernible from other generally longitudinal registrations along the paper record.
 11. A method of transmitting a message from one location to another location both of which are submerged in the sea and many miles apart, comprising at said another location a. continuously sensing acoustic energy in a very low frequency band characterized by long distance propagation by the seawater, b. cyclically scanning and analyzing said band for spectral distribution, c. facsimile recording the spectral distribution in phase with the cyclic scanning and analyzing to present the frequency-time distribution of acoustic energy in said band sensed in the water, d. projecting into the sea at said one location a series of consecutive acoustic signals of sufficient power to reach said another location each characterized by a swept frequency band corresponding to one of a plurality of distinct subdivisions of the sensed band of acoustic energy, e. the time span of each swept frequency signal being such that the signals record generally transverse to and are discernible from registrations of pulse-like transitory noise and registrations of ship noise and other acoustic energy that is comparatively constant in frequency character for extended periods.
 12. A method of signalling between spaced stations in the sea comprising a. projecting into the sea at one of said stations acoustic power characterized by progressively changing frequency from a selected starting frequency to a selected terminating frequency at an approximately regular rate in a predetermined time interval, b. the projected power frequency being below 1,000 cycles per second for propagating the signal over a substantial range and the time interval for the change being at least one minute, c. continuously sensing at the other station acoustic energy reaching the other station between band limits that include the swept frequency of said projected acoustic power, d. driving an elongate paper record longitudinally at a fixed rate and recording along the paper record, using the length of the paper record as a time base and the width of the paper record as a frequency ordinate, the frequency distribution of acoustic energy in said band e. whereby any sensed acoustic energy at relatively constant frequency records along the paper record as longitudinal linear registrations and the swept frequency signal records as a line transverse to the length of the paper record and readily discernible from other registrations on the paper record. 